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September 20, 2013

Soot emissions: a significant contributor to global warming


September 20, 2013

Soot is universally found in the troposphere [the lowest portion of the earth’s atmosphere] and is being labelled as perhaps the second largest contributor to global warming after CO2. Soot is a forcing agent for global warming comparable to the greenhouse gas methane and is one of the deadliest forms of air pollution. It can come in solid, liquid, or gaseous (aerosols) states.
Soot is the common term for a type of particle pollution called PM 2.5 — particulate matter that is 2.5 micrometers in diameter or smaller. Such fine particles are even smaller than dust and can easily enter lungs and bloodstream, causing potential damage in a number of ways.
According to the US Environmental Protection Agency, these fine particles have been linked to a wide range of serious health effects, including premature death, heart attacks, and strokes, as well as acute bronchitis and aggravated asthma among children.
The major source of soot is incomplete combustion of fossil fuels and other organic matter, while the principal sources of soot emissions are coal burning furnaces, coke production processes, wood burning in home fireplaces, open burning of waste and gasoline and diesel-powered engines.
Global warming: Jacobson (2002) study reveals that worldwide soot production is 7-10 billion kilograms annually. Jacobson estimated that because of its ability to absorb and reradiate the sun’s heat, soot may be responsible for 15-30% of global warming.
The study also found that eliminating fossil fuel particulates black carbon and organic matter could eliminate 20-45% of the net global warming within 3-5 years if no other changes occur. Reducing CO2 by a third would do the same but after 50-200 years.
Global average temperature rise: The global average surface temperature has increased about 0.75°C (1.35°F) during the period of extensive instrumental measurements i.e. since the late 1800s. Most of the warming, about 0.5°C (0.9°F), has occurred since 1950.

This can be attributed to economic growth, industrialization, urbanization, improved life style and transportation growth over the globe. Currently, the IPCC projects a warming of 1.0-3.5°C (1.8-6.3°F) by the year 2100.
Hansen (2003) says soot may be responsible for regional as well as global climate change. NASA research into black carbon pollution three years ago suggests that soot emissions may have contributed to flooding in China. The black carbon effect is complex. In regions of India and China sunlight at the surface is reduced due to significant soot concentrations, causing a local surface cooling. However, at higher concentrations, the heating of the air results in surface warming through its influence on atmospheric stability and cloud cover (Hansen, 1997).
Soot in aerosols: According to Hansen (2003) aerosols also cause an indirect climate forcing by changing the properties of cloud drops. Human-made aerosols increase the number of condensation nuclei for cloud droplets formation, thus making the average size of cloud drops smaller. The larger number of smaller drops not only makes the clouds slightly brighter, but also hinders rain. As a result, the average cloud lifetime increases. The brighter long-lasting clouds reduce the amount of sunlight absorbed by the earth, so the indirect effect of aerosols is a negative forcing that causes cooling.
Due to its global influence on climate and tropospheric chemistry while soot is integrated into existing cloud droplets, it leads to a decrease in cloud albedo by absorbing radiation (Ackerman, et al., 2000; Rosenfeld, 2000; Jacobson, 2001). Changes in droplet concentrations have an effect on precipitation patterns. The entrapment of highly absorbing black soot cores within liquid droplets may lead to a local warming, regional and global implications and evaporation of clouds known as the cloud-burning effect (Ackerman, et al., 2000; Rosenfeld, 2000; Jacobson, 2001).
Heat trapping emissions: In its study, the Union of Concerned Scientists (UCS, 1997) mentions that heat-trapping emissions from transportation and other sources — primarily those that burn fossil fuels — have led to an increase in the earth’s temperature. Transportation is considered the source of roughly one-third of all heat-trapping gases released in the US. This is more than most countries releases from all sources combined percentage (Only China, Russia, and Japan have higher total emissions).
Each gallon of diesel fuel burned in a diesel truck engine results in emissions of 22.8 pounds of carbon and other heat-trapping gases. Each gallon of diesel is responsible for additional 2.4 kg of heat-trapping gases at the time of production and delivery. Nationally, heavy trucks emit nearly 400 million metric tons of heat-trapping gases annually, accounting for about 6 percent of US carbon emissions.
Consequences of global warming: According to the UCS, within next 20 years different regions of the world may likely see longer droughts, more coastal flooding, and more frequent extreme weather events. With continued global warming, there could be an increased risk to human health, fisheries, water resources, and severe stress on large forest areas, loss of mountain and coastal-wetland habitats and plants and animals that live there. There could be chances to the expansion rate of deserts, disruption of agriculture, and a rise in sea level of anywhere from 6 to 37.5 inches above the current level with persistent coastal flooding (Gresham, 2000; Washington Post, 1999).
NASA investigations on effects of soot on snow: Soot particles affect climate when they darken snow and ice, causing it to absorb sunlight rather than reflect it (Kirby, 2003). The estimated climate forcing includes the effect of soot on reducing the reflectance of snow and ice (Hansen, 2003).
Why we see glaciers melting even when it is very, very cold in the Northern Hemisphere. This is assumed to be localised heating situations due to dust and black carbon. The research presented at the American Geophysical Union meeting showed that 20pc decrease in the extent of Himalayan glaciers since the 1960s may be partly due to an influx of black carbon from Asian cities.
A NASA research report (Hansen and Nazarenko, 2004) also suggests that black soot emissions alter the way sunlight reflects off snow. According to a simulation, black soot may be responsible for 25% of observed global warming over the past century.
Conclusion: The first step towards realization of soot as one of the major environmental issues could possibly be to gather quality data on soot emissions in regions where this issue has not yet been addressed. This could be followed up by an extensive study on various aspects of soot, such as emission, concentration, transport and deposition to be conducted simultaneously in all continents in order to get a snapshot from global perspective (using NASA satellites, Landsat Thematic Mapper ™ and Satellite Pour d’Observation de la Terre (SPOT).
Homogeneity of data, using automation or manual techniques will be an effort towards generating standard methods for soot measurements. Data and recommendations may be sent to the IPCC for their consideration of the global warming potential of soot.
A review should also be sent to the United Nations Framework Convention on Climate Change (UNFCCC) for their consideration on inclusion of soot, as one of the global warming emissions, in green house gas inventories of the countries. This type of study coupled with calculation of fuel consumption can provide a useful data set for the design of future environmental policies towards maintaining global temperature at a level that curtails sea level rise, forest fires, and acid rains.
The writer is chief chemist at Hydrocarbon Development Institute of Pakistan
[email protected]

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